The Topographic Wake of Transient Landscape Response Forced by Knickpoint Retreat in Post-Orogenic Settings

Transient landscape response to local base-level fall has been well documented in tectonically active mountain ranges. Most of these studies illustrate a relationship between base-level fall (e.g. uplift) and enhanced topographic relief, slope steepness, and sometimes increase rates of erosion, yet many of these ideas have not been applied to post-orogenic landscapes. Locally high topographic relief, steep slopes, and frequent mass wasting characterize the landscapes of the southern Appalachians, an orogen that has been tectonically quiescent for more that 200 Ma, but the fundamental processes responsible for these characteristics remain unclear. We present evidence from the non-glaciated Cullasaja River basin of southwestern North Carolina that highlight the existence of a transient spatial relationship between knickpoints present along the fluvial network and adjacent hillslopes. The Cullasaja River basin was chosen for this study because of its relatively uniform lithology, frequent debris flows, and the availability of high-resolution airborne lidar DEMs. Topographic metrics (e.g. relief, slope gradient, hypsometric integral, ect.) extracted from 14 tributary basins exhibit significant downstream increases below the current position from major knickpoints and begin to decay approaching the mouth the Cullasaja, resembling a wake in topography. Furthermore, hillslope activity (landslide frequency) extracted from the same tributary drainage basins show continued downstream increases below the knickpoints suggesting that the hillslopes are responding to channel forcing by accelerated rates of erosion. Our results suggest that the contemporary high relief and steep slopes of the southern Appalachians are the result of a series of local base-level falls induced by the passage of knickpoints, of unknown origin, to which the landscape responds presumably by increased rates of hillslope denudation often manifested as an increased frequency of mass wasting events. Further research is needed to quantify the drivers behind knickpoint formation, their retreat rates, and possible spatial and temporal changes in the rates of hillslope erosion throughout the Cullasaja River basin.